Supported grain design for high acceleration rocket motors

ABSTRACT

1. A solid propellant rocket motor comprising a case having a generally cylindrical chamber section, a forward closure, and a nozzle section; FOR CARRYING SOLID PROPELLANT GRAIN, A PLURALITY OF STRUCTURAL REINFORCEMENTS CONSTITUTING RIBS EXTENDING LONGITUDINALLY IN SAID CYLINDRICAL CHAMBER, DISPOSED WITH THE HEIGHTS OF SAID RIBS POSITIONED GENERALLY RADIALLY OF SAID CHAMBER AND SPACED CIRCUMFERENTIALLY FROM ONE ANOTHER AROUND THE INTERIOR OF SAID CHAMBER; MEANS CONNECTING SAID REINFORCEMENT TO SAID CASE AT THE SAID FORWARD CLOSURE SO THAT ACCELERATION LOADS PLACE SAID REINFORCEMENTS IN TENSION; AND SOLID PROPELLANT GRAIN BONDED TO SAID REINFORCEMENTS.

United States Patent Iwanciow et a1.

[451 May 23, 1972 [54] SUPPORTED GRAIN DESIGN FOR HIGH ACCELERATIONROCKET MOTORS [72] Inventors: Bernard L. Iwanclow, Sunnyvale; Robert A.Chase, Los Altos Hills, both of Calif.

[73] Assignee: The United States of America as represented by theSecmtaiy of the Navy [22] Filed: June30, 1965 [21] App]. No.: 469,974

[52] US. Cl ..60/255, 102/102 [51] Int. Cl ..F02k 9/04 [58] Field ofSearch .60/35.6, 255; 102/98, 102

[56] References Cited UNITED STATES PATENTS 2,995,091 8/1961 Haymes etal. 102/98 3,048,968 8/1962 Hutchinson..... 3,090,196 5/1963 Brewer..102/98 X OTHER PUBLICATIONS Recent Advances in Solid Propellant GrainDesign," 60- 35.6 R.S. by Jean A Vandeakerckhove, A.R,S. Journal, Vol.29, No. 7, July 1959, pages 483- 491.

Primary Examiner-Samuel Feinberg Att0rney-R. l. Tompkins and E. F.Johnston EXEMPLARY CLAIM 1 A solid propellant rocket motor comprising acase having a generally cylindrical chamber section, a forward closure,and a nozzle section;

for carrying solid propellant grain, a plurality of structuralreinforcements constituting ribs extending longitudinally in saidcylindrical chamber, disposed with the heights of said ribs positionedgenerally radially of said chamber and spaced circumferentially from oneanother around the interior of said chamber;

means connecting said reinforcement to said case at the said forwardclosure so that acceleration loads place said reinforcements in tension;and

solid propellant grain bonded to said reinforcements.

10 Claims, 4 Drawing Figures PATENTEBMAY 2 3 I872 INVENTORS BERNARD L.IWANCIOW ROBERT A. CHASE SUPPORTED GRAIN DESIGN FOR HIGH ACCELERATIONROCKET MOTORS This invention relates to a supported grain design forhigh acceleration rocket motors.

The general problem of the development of an anti-missile missile placesan unusually high acceleration requirement on a propulsion system. Theobject of this invention is to provide a rocket motor for use in suchmissiles in which a solid propellant grain can be subjected withoutstructural failure to accelerations of the order of from some 300 gs toperhaps over 2,000 gs.

Other objects and many of the attendant advantages of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is an axonometric exterior view of a rocket motor using apreferred embodiment of this invention, the view being partially brokenaway to show the interior construction.

FIG. 2 is a cross section taken along the line 22 in FIG. 1.

FIG. 3 is an enlarged fragmentary exploded view of the forward end of agrain-supporting reinforcement and a spacer visible in the broken-awayportion of the forward end of the rocket motor of FIG. 1; and

FIG. 4 is a fragmentary cross-sectional view taken perpendicular to thelongitudinal axis of the motor of FIG. 1 showing means for retainingreinforcements carrying solid propellant against displacement relativeto the motor case except in the longitudinal direction.

Because of the high accelerations required in an anti-missile missile,the booster must be designed to discharge propellant gases through thenozzle at a high rate of discharge as governed by the thrust equation,

F in I where F thrust r mass rate of discharge l= specific impulse ofthe propellant The equations governing the rate of generation ofpropellant gases and the rate of discharge of gases through the nozzlewhich are equal at equilibrium are:

rh P,Sr C P A, where P, propellant density S surface area of propellantbeing burned r burning rate of propellant C,, discharge coefficient P,chamber pressure A, thrust area To attain the required value of in,either a high burning rate propellant in a grain configurationcharacterized by a small surface area or a low burning rate propellantwith a high surface area can be utilized.

The state of the art of propellant technology is such that the availableburning rates are not high enough to develop the required propulsionsystems with conventional geometries; therefore, the grain design ofthis invention was developed to utilize conventional propellants with anovel grain geometry. This invention is based upon the use of asupporting structure which carries the acceleration load that wouldotherwise, in conventional rocket motors, be imposed upon thepropellant. The substitution of this supporting structure for theload-carrying function in place of the propellant, eliminates the needfor a high strength propellant.

For a preferred embodiment of the invention, reference is now made tothe drawing. In FIG. 1 there is shown generally at 2 a rocket motorembodying the invention. The casing 4 can be made of steel, fiberglass,or any other suitable material and comprises a generally cylindricalmajor portion forming a cylindrical chamber, a forward closure or headend 6, conveniently generally hemispherical, and a nozzle 8. Thepropellant which can be conventional solid propellant grain such as thatmade of binder, oxidizer (typically ammonium perchlorate) and catalyst,is bonded to and carried by a plurality of structural supports orreinforcements. One group of such structural reinforcements or supportsis made up of a plurality of ribs 12 each of which may be of anysuitable shape but are shown for illustration only to be generally inthe form of an I beam. The propellant 10 is bonded to the ribs in thechannelshaped cavities in the I beams constituted by the webs andflanges of the I beams. The ribs extend generally longitudinally alongthe length of the interior of the motor case and are disposed with theheights of the ribs positioned generally radially of the case as seenmost clearly in the cross-sectional view in FIG. 2. The l-beam shapedribs are fastened to the motor case in the region of the forward closure6. This can be accomplished in any convenient fashion. For example, theweb of the I beam rib 12 can be extended as seen at 14 in FIG. 3 andconfigured to fit the contour of the forward closure 6 as indicated bythe curved portion 16. The propellant structural support orreinforcement 12 is placed in the motor case with the web extension 14of the I beam reinforcement 12 fitting against the interior surface ofthe forward closure 6 as seen in FIGS. 1 and 2 and the supports carryingthe propellant are then fastened to the case by potting the extensions14 in the forward closure 6 with any suitable bonding material such asepoxy resin.

To increase the amount of propellant which can be loaded into the motor,smaller propellant strips, generally indicated at 18, are used topartially fill the wedge-shaped space 20 which is formed, betweenadjacent propellant slabs of the relatively larger size carried on thereinforcement 12, in the region of the cylindrical chamber where thechamber is of relatively greater diameter. The slabs or strips 18 ofpropellant are bonded to structural supports 22 in the same manner aspropellant strips 10 are bonded to support 12. The only difference isthat the height of the I beam or similar section is less in case of theelements 22 than in the case of the elements 12. An extension of the webof each reinforcement 22, similar to the extension 14 of the support 12,is formed to facilitate fastening each support 22 to the motor case. Tomaintain the supports 12 and 22 properly spaced from one another and toreduce the amount of potting or bonding material necessary to bond thesupports 12 and 22 to the forward closure 6, spacers 24 are insertedbetween the web extensions of the member 12 and the web extensions ofthe members 22 on each side of each member 12 as seen especially inFIG. 1. The potting or bonding material is poured or otherwise placedinto the forward closure after all the supports 12 and 22 and thespacers 24 are in place.

Any other suitable means can be used to fasten to the motor case 4 thesupports 12 and 22 so long as they are of the type which serves totransmit from the case to the supports tensile forces directedlongitudinally of the case. For example, a spider can be bonded, welded,or bolted to the forward end of the case and projections on the members12 and 22 can be used to engage the spider. The members 12 and 22 can bebolted or otherwise fastened to the spider.

The elements 12 and 22 can be bonded to the case 4 essentially alongtheir entire length or, if preferred, they can be held against the case4 in such a manner to allow only longitudinal displacement relative tothe case 4 by any convenient means such as T-shaped lugs 26 fastened tothe case 4 and engaging flanges of the elements 12 and 22 as seen inFIG. 4. Such an arrangement accommodates relative displacement of thecase and reinforcements occasioned by thermal expansionv The elements 12and 22 can be made of any suitable heat resistant, high tensile strengthmaterial such as structural fiberglass sheet.

The motor may be provided in known manner with molded graphite cloth 28in the nozzle to resist high temperatures. The propellant can be ignitedby a conventional igniter tube 30 carried by the nozzle or by thelaunching facility.

The thickness of the grain support structure exemplified by theI-beam-shaped ribs is related to the dimensions of the grain, thetensile strength of the support material, the propellant density, thedimensions of the propellant slabs and the acceleration level at thestart of boost. The thickness can be cal culated from:

h (2p 1 hg)/S where h thickness of support structure S tensile strengthof support structure material p propellant density l= length of grainslab h thickness of propellant slab g acceleration at start of boost Anexample is calculated for the following set of parameters:

p 0.063 No./in

I 100 in h 0.5 in

g 500 gs h 0.045 in.

S 70,000 psi (fiberglass) The maximum shear stress at thepropellant-support structure interface is determined by the propellantthickness, propellant density and acceleration and is given by theexpression:

Thus for the example given, the structure support is under a tensilestress of 70,000 psi at the start of boost while shear stress at thepropellant-support interface is only:

o',,,,, 0.063 X 0.5 X 500 15.75 psi shear This shear stress is wellbelow the nominal 50 psi shear strength of conventional propellants.

As stated previously, a propellant with a low burning rate requires alarge surface area to obtain the necessary mass discharge rates. Use ofthe multiple slab configuration of this invention with thin webs assuresthis high surface area. Tests using simulated high g forces have shownthat a test specimen with the grain configured according to thisinvention did not fail when forces corresponding to 2,200 gs wereimpressed upon it.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A solid propellant rocket motor comprising a case having a generallycylindrical chamber section, a forward closure, and a nozzle section;

for carrying solid propellant grain, a plurality of structuralreinforcements constituting ribs extending longitudinally in saidcylindrical chamber, disposed with the heights of said ribs positionedgenerally radially of said chamber and spaced circumferentially from oneanother around the interior of said chamber;

means connecting said reinforcements to said case at the said forwardclosure so that acceleration loads place said reinforcements in tension;and

solid propellant grain bonded to said reinforcements.

2. The motor of claim 1 wherein certain of the reinforcements haverelatively greater heights and certain others of said reinforcementshave relatively lesser heights and wherein reinforcements of relativelylesser heights are disposed between reinforcements of relatively greaterheights in regions of said cylindrical chamber where said chamber is ofrelatively greater diameter.

3. The motor of claim 1 wherein said reinforcements are generally in theform of I beams and said solid propellant grain is bonded to saidreinforcements in the channel-shaped cavities in said I beamsconstituted by the webs and flanges of said I beams.

4. The motor of claim 3 wherein the means connecting said reinforcementsto said case includes extensions of the webs of said I beams configuredto fit the contour of said forward closure and bonded to said forwardclosure. I

The motor of claim 1 wherein said connecting means connects saidreinforcements to said case in the region of said forward closure andwherein means are provided, spaced, in a longitudinal direction fromsaid forward closure, to essentially retain said reinforcements fromdisplacement relative to said case except in the longitudinal direction,whereby to allow for relative displacement of case and reinforcementsoccasioned by thermal expansion.

6. The motor of claim 1 wherein said reinforcements are configured tofit the contour of said forward closure and bonded to said forwardclosure.

7. The motor of claim 1 wherein said reinforcements are generally in theform of l beams and are of high tensile strength fiberglass material.

8. The motor of claim 1 wherein spacers are inserted between saidreinforcements in said forward closure for maintaining saidreinforcements properly spaced from one another.

9. The motor of claim 1 wherein holding means are placed between saidreinforcements for holding said reinforcements to the case and forallowing only longitudinal displacement.

10. The motor of claim 9 wherein said holding and spacing means compriseT-shaped lugs attached to the case and engaging said reinforcements.

1. A solid propellant rocket motor comprising a case having a generallycylindrical chamber section, a forward closure, and a nozzle section;for carrying solid propellant grain, a plurality of structuralreinforcements constituting ribs extending longitudinally in saidcylindrical chamber, disposed with the heights of said ribs positionedgEnerally radially of said chamber and spaced circumferentially from oneanother around the interior of said chamber; means connecting saidreinforcements to said case at the said forward closure so thatacceleration loads place said reinforcements in tension; and solidpropellant grain bonded to said reinforcements.
 2. The motor of claim 1wherein certain of the reinforcements have relatively greater heightsand certain others of said reinforcements have relatively lesser heightsand wherein reinforcements of relatively lesser heights are disposedbetween reinforcements of relatively greater heights in regions of saidcylindrical chamber where said chamber is of relatively greaterdiameter.
 3. The motor of claim 1 wherein said reinforcements aregenerally in the form of I beams and said solid propellant grain isbonded to said reinforcements in the channel-shaped cavities in said Ibeams constituted by the webs and flanges of said I beams.
 4. The motorof claim 3 wherein the means connecting said reinforcements to said caseincludes extensions of the webs of said I beams configured to fit thecontour of said forward closure and bonded to said forward closure. 5.The motor of claim 1 wherein said connecting means connects saidreinforcements to said case in the region of said forward closure andwherein means are provided, spaced, in a longitudinal direction fromsaid forward closure, to essentially retain said reinforcements fromdisplacement relative to said case except in the longitudinal direction,whereby to allow for relative displacement of case and reinforcementsoccasioned by thermal expansion.
 6. The motor of claim 1 wherein saidreinforcements are configured to fit the contour of said forward closureand bonded to said forward closure.
 7. The motor of claim 1 wherein saidreinforcements are generally in the form of I beams and are of hightensile strength fiberglass material.
 8. The motor of claim 1 whereinspacers are inserted between said reinforcements in said forward closurefor maintaining said reinforcements properly spaced from one another. 9.The motor of claim 1 wherein holding means are placed between saidreinforcements for holding said reinforcements to the case and forallowing only longitudinal displacement.
 10. The motor of claim 9wherein said holding and spacing means comprise T-shaped lugs attachedto the case and engaging said reinforcements.